1. Field of the Invention
The present invention relates to a rotary-anode type X-ray tube and, more particularly, to an improvement in the structure of a bearing for supporting a rotary-anode type X-ray tube.
2. Description of the Related Art
As is known, in a rotary-anode type X ray tube, a disk-like anode target is supported by a rotary structure and a stationary shaft having a bearing portion therebetween, and an electron beam emitted from a cathode is radiated on the anode target while the anode target is rotated at a high speed by a rotating magnetic field generated by energizing the electromagnetic coil of a stator arranged outside a vacuum envelope, thus irradiating X-rays. The bearing portion is constituted by a rolling bearing, such as a ball bearing, or a dynamic pressure type sliding bearing which has bearing surfaces with spiral grooves and uses a metal lubricant consisting of, e.g., gallium (Ga) or a gallium-indium-tin (Ga-In-Sn) alloy. Rotary-anode type X-ray tubes using the latter bearing are disclosed in, e.g., Published Examined Japanese Patent Application No. 60-21463 and Published Unexamined Japanese Patent Application Nos. 60-97536, 60-117531, and 62-287555.
In the rotary-anode type X-ray tubes disclosed in the above-mentioned official gazettes, molybdenum (Mo) or an Mo alloy, or tungsten (W) or a W alloy is used as a material for a rotary structure and a stationary shaft constituting a sliding bearing. However, a bearing surface consisting of such a metal is easily oxidized in the air in an assembly process, and the resulting oxide film causes a deterioration in wettability between the bearing surface and a liquid metal lubricant consisting of a Ga alloy. For this reason, a special process is required to remove the oxide film formed on the bearing surface, making the assembly process cumbersome. Moreover, a reliable dynamic pressure type sliding bearing function may not be obtained.